CN110139728B

CN110139728B - Pulse laser processing method for obtaining diamond with smooth transparent surface

Info

Publication number: CN110139728B
Application number: CN201780080939.5A
Authority: CN
Inventors: E·鲁奥戴尔; G·威克; T·普利森
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2016-12-28
Filing date: 2017-12-21
Publication date: 2021-05-07
Anticipated expiration: 2037-12-21
Also published as: US20200122268A1; US11446759B2; FR3061052A1; FR3061052B1; EP3562615B1; CN110139728A; WO2018122503A1; EP3562615A1

Abstract

In order to machine a diamond (2) by means of a pulsed laser, the diamond (2) is placed in a container (20) containing a transparent liquid (3) whose level is located on the surface of the diamond (2) to be machined At least 100 microns above, this transparent liquid (3) may contain a surfactant additive in an amount at least equal to 2 to 10% by mass. The laser source (10) is then activated to apply a laser beam (1) to the surface to be machined, the laser beam (1) having pulses with a duration equal to at most 1 microsecond at a repetition frequency of at most 5 kHz, and in the diamond (2) ) and the laser source (10), a relative scan is controlled transverse to the laser beam (1) and in the axial depth direction, the amplitude and orientation of which is determined by the shape of the machining performed in the diamond (2).

Description

Pulse laser processing method for obtaining diamond with smooth transparent surface

Technical Field

The present invention relates to a laser machining method of diamond enabling a smooth transparent surface to be obtained.

Field of application

The invention finds particular application in the field of diamond engineering for physical experiments. Companies such as Almax Easylab, Technodiamat, or Element six provide diamonds drilled with holes, cones, or grooves for physical experiments.

Other applications of laser cutting diamond involve the production of diamond lenses (diamond lenses are increasingly used, particularly on large synchrotrons), machining of diamond substrates for optical, electronic or microfluidic, jewelry processes (by laser cutting or pre-cutting of diamond, polishing of diamond by laser).

However, as described below, the surface obtained is not actually transparent; and obtaining a transparent surface opens up new possibilities for product commercialization.

Background

Prior art for machining diamond by laser

Traditionally, cutting of diamonds is done mechanically by using the natural cleavage plane of the diamond; the surface is then polished until a roughness low enough to make the diamond transparent is obtained.

Lasers have long been used to ablate, cut or polish diamond because of the significant time savings and freedom in cutting shape. This is because the commonly used laser can be focused on a very small area, typically 1 micron wide, and can ablate diamond in any particular form, not just along the crystal cleavage plane. However, prior art laser ablation has the major disadvantage of not only generating debris, but also converting a somewhat large portion of the machined surface into black graphite. Post-processing by chemical reaction or mechanical polishing is then still required.

Many experts, research groups and entrepreneurs have studied the machining problem of diamond and the machining method proposed as being able to limit the appearance of graphite combines two main elements:

using lasers with ultra-short pulse duration (typically less than 10ns) [ see reference 1],

processing the diamond in a liquid medium (typically water or an organic solvent).

However, the surface obtained by laser machining still contains a more or less large amount of graphite. A recent review (2013) of laser machining of diamond in air [1] (page 423) explains: the problem is relatively complex because the surface will have poor smoothness when ablated by the laser and will be covered by a graphitized layer.

By working in water, some sources mention obtaining a "thermally unaffected surface" or a "graphite-free surface". However, when the data is presented, it is found that a small layer of graphite is still present. Furthermore, the transparency of the surface has never been mentioned or shown; in various recent studies [ references 2 to 4], which have main experimental characteristics and in which the specificity of diamond is not always properly considered, only channels are excavated and there is no data on the transparency properties of the machined surface. Although it appears that the volume of the graphite produced is reduced by processing in water, the following conclusions cannot be drawn: graphite is not present and, in particular, the surface obtained is sufficiently smooth and free of graphite so that the surface is transparent without any subsequent finishing treatment.

On the other hand, if the documents available from commercial companies that laser process diamond are examined, it can be concluded that the surface is not smooth enough and/or does not contain graphite to result in transparency. Thus, for example, the company "Synova" proposes a laser machining in which a laser beam is guided in a water jet; in addition to the fact that the size of the laser beam is itself too large for micro-machining applications, it is also noted in the commercial brochure: relating to the surface quality of the sample: "very smooth sidewalls with an extremely thin black layer that is easily removed".

Similarly, it was found that the machined diamond sold by Almax Easylab, a company that commercialized laser machined diamond for physical experiments, could not meet this condition of being sufficiently smooth and free of graphite to be transparent.

In any event, it is very useful to have a single crystal diamond that: the surface of the single crystal diamond is processed using a laser in such a manner that subsequent processing is not required, and the surface is transparent.

It is noteworthy that it is currently possible to obtain transparent and very smooth surfaces on single-crystal diamond, but this is achieved by using processing techniques without laser, for example using a focused ion beam, but such techniques have the disadvantage of having long implementation times and very high costs.

It has been mentioned above that it may be necessary to apply a subsequent treatment to remove the graphite after processing, typically by conventional mechanical polishing or by chemical or photochemical reactions (e.g. involving laser reactions induced by UV lasers or ozone [5 ]). However, this post-processing is only successfully used in the case of very small (<1 micron) depth processing. In addition to this, graphite generated during the machining disturbs the machining itself, and the obtained shape has large irregularities.

It thus appears that there is currently no diamond laser machining technique at a significant depth (greater than 1 micron, even greater than 5 microns) that makes it possible to obtain a machined surface that is sufficiently smooth and transparent, free of graphite, while having suitable dimensions, preferably without additional treatment.

Disclosure of Invention

Presentation of the invention

The present invention provides a method for direct deep cutting of diamond with a laser that leaves a surface transparent to visible light without post-treatment.

The invention provides a method for processing diamond by means of a pulsed laser, according to which:

-placing the diamond in a container containing a transparent liquid, the level of which is at least 100 microns above the surface of the diamond to be processed, activating a laser source to apply a laser beam to the surface to be processed, the laser beam having pulses with a repetition rate of at most 5kHz and a duration at most equal to 1 microsecond,

-controlling, between the diamond and the laser source, a relative scan transverse to the laser beam and in the axial depth direction, the amplitude and orientation of which are determined by the shape of the machining carried out in the diamond.

The method is based on several elements. According to the teachings of the state of the art commented above, it combines the use of ultrashort lasers (i.e. with ultrashort pulses) with the presence of a liquid in which the processing (or ablation) is carried out.

However, this method involves conditions regarding the repetition frequency of the ultrashort pulses, advantageously combined with conditions regarding the liquid in which the diamond to be processed is immersed.

Rather, the frequency of the laser pulses must be lower than in the prior art, and one might instead believe that, in order to have a very smooth surface, a high repetition rate is required to process as continuously as possible at a given speed of movement.

It has furthermore been found that the presence of surfactant in the liquid surprisingly has a beneficial effect on the surface quality obtained, even leading to transparency.

The invention is based on the adjustment of the repetition rate of the laser, that is to say the time interval between successive pulses of material removal, advantageously supplemented by the introduction of a surfactant. This repetition rate must be less than a few kilohertz (typically less than 5kHz, even less than 2kHz) and ideally less than 1kHz or even 500 Hz. This amounts to leaving a time at least equal to about one millisecond (or even several milliseconds) between two successive pulses.

The polarization of the laser light is chosen to be circular or elliptical if not circular, but as close to circular as possible.

The introduction of the surfactant improves the quality of the results obtained, since it reduces the meniscus present at the surface of the liquid and makes the processed surface smoother. The presence of the surfactant also enables the maintenance of the processing quality at higher laser repetition frequencies above 500 hz, and up to frequencies of several kilohertz (typically 5khz), while maintaining the transparent nature of the processed surface.

According to an advantageous characteristic of the invention:

the liquid (in particular, typically pure and/or demineralized water, or an aqueous solution) comprises a surfactant additive; it may be in particular a surfactant additive of the polyoxyethylene type.

The liquid also contains mineral salts in a content at most equal to 10% by mass; it may especially be sodium chloride.

The liquid is moved by mechanical agitation.

The level of liquid above the surface to be processed is at least equal to 100 microns (preferably 200 microns) and at most equal to 3mm (preferably 1 mm).

For a repetition frequency of 100Hz to 1kHz, even 200 and 800Hz, the pulse duration is between 100 femtoseconds and 100 picoseconds, preferably at most equal to 1 picosecond or even 500 femtoseconds.

The wavelength of the laser beam is 500-530 nm.

The polarization of the laser light is circular.

The scan is controlled axially, transverse to the beam and between two transverse scan cycles, in steps at most equal to 10 microns or even 1 micron.

The processing is performed in one or more cycles to a depth of at least 10 microns.

The focusing objective is an immersion objective which is directly immersed in the liquid used.

Drawings

Description of the invention

The objects, characteristics and advantages of the present invention are apparent from the following description, which is given by way of illustrative and non-limiting example with reference to the accompanying drawings, in which:

figure 1 is a schematic diagram of a device for implementing the invention,

FIG. 2 is a scanning electron microscope image of transparent pits (cuvette) machined in a diamond according to the method of the invention,

FIG. 3 is a graph showing the Raman spectra obtained on the raw surface of the diamond and at the bottom of the pits according to FIG. 2,

FIG. 4 is an interference pattern obtained with white light between the bottom of such a pit and the rough surface of the diamond,

FIG. 5 is a spectral signal recorded for a sample placed in such a pit, and

fig. 6 is an optical image of a paper fiber placed under a diamond in which a plurality of pits have been made, these pits being obtained in a classical manner (opaque) or according to the invention (transparent).

Detailed Description

As schematically shown in FIG. 1, the method of the invention comprises the use of a laser source 10 of the pulsed type, with circular or near-circular polarization, emitting for a duration of at most 5kHz, preferably at most equal to 1kHz, with a repetition frequencydA micropulse at most equal to a microsecond, preferably at most equal to 500 picoseconds (even less than 100 picoseconds, even less than picoseconds).

This laser source is connected to a focusing objective 11, enabling the focus of the laser beam 1 on the sample marked 2 to be adjusted. In the embodiment shown, the focusing optics is an objective lens operating in air; as a variant not shown, it may be an "immersion" objective, i.e. designed to be immersed directly in the liquid (in which case the beam coming from this objective propagates completely in the liquid).

This sample 2 is placed in a container 20, the container 20 containing a transparent liquid 3, the sample being immersed in the transparent liquid and positioned facing the focusing optics. In practice, the height position of this optics is adjustable relative to the height of the sample (that is, the position of one and/or the other is adjustable relative to each other), which is schematically shown in fig. 1 by the vertical double arrow. This adjustment makes it possible to focus the laser beam on the surface to be processed of the sample.

The sample 2 is a natural or synthetic single-crystal or polycrystalline diamond (for example a nanocrystal) which is advantageously fixed to a support (which here consists of the bottom of the container), for example by gluing.

If the objective used is an immersion objective, the level of the liquid above the sample is at least equal to the distance between the objective and the surface of the sample when the sample is located at the focal point of the objective, that is to say when the laser light is focused by the objective on the surface of the sample.

If the objective lens used is operated in air, the level of the liquid used should be adjusted according to the container, the focusing distance of the objective lens and the processing duration. The level of the liquid should typically be greater than 100 microns, preferably at least equal to 200 microns. The level of the liquid that must not be exceeded is determined by the appearance of the motion of the surface of the liquid (typically small waves are formed, which is detrimental to the quality of the process). A liquid height of a few millimetres (typically 3mm) may be used in a suitable container, such as a small cup, in which a piece of metal is placed to disrupt the formation of waves. A higher limit is the distance to the objective, which of course cannot be exceeded if the objective is not an immersion objective. Such a distance may thus be chosen in particular to be 100 micrometers to 3mm, for example 200 micrometers to about 1 mm.

In practice, the wavelength is advantageously chosen according to the liquid used in order to minimize its absorption by the liquid before reaching the surface to be processed.

The power of the laser used is chosen so that the energy density of the beam exceeds the ablation threshold of the diamond in the liquid (typically 5 joules per square centimeter in the case of water); in practice, an energy density of 80 joules per square centimeter gives very good results (up and down by 5 to 10J/cm²)。

Such transparent liquids typically consist essentially of water, which in practice is demineralized water.

However, as a variant, this transparent liquid may also be an aqueous solution comprising an acid or a base, for example potassium hydroxide in the form of an aqueous solution, or hydrogen peroxide, or even a transparent hydrocarbon.

Such transparent liquids advantageously have surfactant properties; it is thus advantageously water containing a surfactant additive. These surfactant properties have been shown to improve the processing conditions and in particular enable a significant increase in the repetition rate of the laser (typically 500-5kHz) while maintaining the transparency of the processed surface (no deposition of graphite).

This may thus involve additives of the type having an ethylene oxide chain (i.e. polyoxyethylene).

This relates, for example, to additive C₁₄H₂₂O(C₂H₄)_nFor example, under the trademark Triton X-100 by Dow Chemical Company

Additives sold in the meat of the animals; such nonionic surfactant additives have the advantage of being transparent, colorless and non-foaming.

However, the benefits of the surfactant effect take advantage of the use of a few drops of common dishwashing liquid (e.g., Paic, trademark)

Sale) were also observed, thus indicating all types of surfactants (non-ionic, e.g., Triton X-100)

In the form of an "egg", or substantially anionic, e.g. < Paic >

In the cross section) produces the effects sought for diamond laser machining.

However, the surfactant additive chosen is advantageously chosen by its transparency properties and the absence of foaming ability, so as to interfere as little as possible with the passage of laser light in the liquid. It is present in an amount sufficient to obtain a detergent effect, but not exceeding a threshold which would reduce the clarity of the liquid, for Triton X-100

The concentration is usually 0.05 to 0.2% by volume.

Agitation of the liquid (preferably gentle so as not to produce significant surface movement) has been shown to have a beneficial effect on the process, the same as the introduction of surfactant, i.e. improved process, enabling increased repetition frequency of the laser pulses (for the same process quality).

The presence of mineral salts (for example sodium chloride) has also proved to have a beneficial effect on the elimination of any graphite deposits and on the possible depth of machining and on the minimization of the roughness of the machined surface.

Such salts may be present in an amount of up to 10%, preferably 3-7% by mass;

the machining of diamonds consists in practice in performing a relative scan between a laser source and the sample, the size of the impact of the laser beam on the sample being substantially smaller than the size of the area to be drilled; as a non-limiting example, if the area to be processed has a typical size of 100 microns (in particular, this is for example the length of one side of a polygon, or the maximum size), a beam focused on 1 square micron would be used; if the area has a characteristic dimension of one millimeter, the beam may be focused only on 10 square microns. The amplitude and orientation of the scan define the geometry of the machining region.

The energy and scanning speed of the beam define the depth at which holes can be drilled per scanning cycle; this depth is typically about 1-10 microns, or at least greater than 0.1 microns.

This scanning may be obtained by moving the focusing optics relative to the sample or by moving the sample under the focusing optics (or a combination of these movements); this is schematically illustrated by the crossed horizontal double arrows shown close to the container support. These movements are performed, for example, using a displacement plate for moving the sample and/or by a galvanometer head for moving the laser beam relative to the sample.

It should be noted that the concept of repetition frequency is locally defined with respect to the successive processing of adjacent regions; thus, the scanning can be performed in parallel along two different tracks, for example separated by a distance equal to at least about ten times the size of the processing speed of light (for example two parallel tracks or two portions of the same track), with the possibility of rapidly changing the orientation of the beam by suitably controlling the galvanometer head.

FIG. 2 shows the result of machining a pit in a diamond; the pit walls have stripes, which appear to be due to successive scan cycles. In contrast, the bottom appears very smooth, with no slightest streaks detectable at this magnification (pits are tens of microns wide in the embodiment considered).

Visual inspection of the pits also made it possible to note that the machined surface was transparent when illuminated with transmitted light, which helped to confirm the very smooth nature of the pit bottom and also confirmed the absence of any graphite deposits during machining.

The quality of the results is characterized by figures 3-6.

Thus, fig. 3 shows a spectrum of a raman spectrum of a surface without a graphite feature signal. The grey curve (denoted a) is the raman spectrum of the natural diamond before laser cutting and the black curve (denoted B) is the spectrum of the diamond at the bottom position of a pit drilled to a depth of 20 microns with a laser. 1600cm^-1The nearby ridges are due to the localized presence of nanocrystalline diamond [6]. Thus, no graphite is formed in the process according to the method of the invention.

The transparency of the pit bottom surface can be confirmed by fig. 4, which shows a white light interference spectrum obtained between the unprocessed reference diamond surface and the surface processed by our method; the possibility of observing interference fringes indicates the transparency of the pit bottom surface and also its perfect flatness. The transparency of the surface processed according to the method of the invention thus makes it possible to carry out spectroscopic investigations by passing a laser through the pits.

This transparency is confirmed in fig. 5, which is a spectral curve obtained by observing foreign substances through a surface processed with laser according to the present invention. More specifically, the work surface is a small pit as shown in fig. 2. In which a ruby is placed. The spectral signal presented is obtained through the diamond, that is, the laser light excited and the signal emitted by the ruby pass through the machining surface (from the back of the sample in fig. 2, and towards the back of the sample, respectively). The transparency properties of the process were found to enable a spectral signal to be obtained through the process surface.

The photograph of fig. 6 shows a single crystal diamond in which various pits have been machined and under which paper fibers are placed: pits machined by conventional methods can be distinguished where a laser is used but in air, that is, without liquid (they appear black and nothing can be seen through the underlying portion of the diamond); and transparent pits, both obtained by the method of the invention, enable the underlying paper fibers to be seen.

It can thus be confirmed that the method of the present invention enables to obtain a transparent working surface free of black graphite. The surface is also smooth enough to enable viewing of the object through the very small pits obtained using this method. It is also possible to perform spectral measurements (in particular laser light, or by absorbing white or infrared light) through the surface, or even to perform interferometric experiments.

It is important to point out that this method enables to use the speed of the laser to diversify the possible processing geometries, by avoiding the necessity of a chemical or mechanical post-treatment after the processing. In particular, the invention makes it possible to machine recesses, such as the pits described above, with laser light, while obtaining the characteristics of cleanliness and geometry, ensuring that the machined surface is transparent. It can be readily appreciated that if the graphite deposit forms in a pit (such as the pit of fig. 2), subsequent processing intended to eliminate it will be difficult to perform and will certainly change the machining dimensions.

By way of example, the invention has been implemented under the following operating conditions, which enable the pits described above and represented in fig. 6 to be obtained:

a laser marketed under the trade name Amplitude sys mes and under the model SATSUMA provides pulses of wavelength 515nm of 300 femtoseconds, using an energy per pulse of 5 microjoules, with a repetition rate of 500 hz and using circular polarization.

-a focusing objective lens: an objective lens with a magnification of x10, under the trade mark Thorlabs, has a numerical aperture of 0.25.

Horizontal scanning of the sample, driven by a displacement plate under the trademark Jenny-Science, with rows spaced by 0.8 microns and vertical drops in steps of 0.5 microns, the entire processing system having been integrated and interfaced by the company OPTEC.

The liquids used: based on a solution of 4mL of demineralized water, 0.2 g of sodium chloride and 0.1 mL of triton 100 (surfactant) (diluted to 5% in demineralized water); the liquid height in a 2cm diameter vessel is about 0.7 mm.

It will be appreciated that since these conditions have resulted in optimum processing, it is within the ability of those skilled in the art to derive the teaching to obtain further optimum trade-offs by varying one or the other parameter.

Thus, in particular, it may be advantageous:

-varying the pulse duration between 100 femtoseconds and tens of picoseconds for a repetition frequency of 200 Hz-1kHz,

-selecting a wavelength from near infrared to near UV,

-modifying the scanning step; it will be appreciated that the more the beam is focused on a small area, the smaller the selectable step size, for example about half the beam diameter,

using an immersion objective directly immersed in the process liquid.

The method is particularly useful for machining diamond to a cumulative depth of at least about ten microns.

The above embodiments relate to single crystal diamond. As a variant, the method may be carried out with polycrystalline diamond, for example nanocrystalline diamond (it should be noted that in this case, it is important from a practical point of view that mechanical polishing is not effective); no difference was found between natural diamond and synthetic diamond.

Reference to the literature

[1]“Optical engineering of diamond”,R.P.Mildren and J.R.Rabeau,Wiley-CH,(Chapter 12:Laser micro-and nanoprocessing of diamond materials),(2013)。

[2]“Laser processing of diamonds and sintered c-BN in liquid”,H.Miyazawa and M.Murakawa,New diamond and frontier technology,10,3200(2001)。

[3]“Laser-assisted etching of diamonds in air and in liquid media”,G.A.Shafeev,E.D.Obraztsova and S.M.Pimenov Mater.Sci.Eng.B-Solid State Mater.Adv.Technol.46,129–132(1997)。

[4]“Underwater and water-assisted laser processing:Part2–Etching,cutting and rarely used methods”,A.Kruusing,Optics and lasers Engineering,41,333(2004)。

[5]“Laser patterning of diamond part II surface nondiamond carbon formation and its removal”,Smedley et al,Journal of applied Physics,105,123108(2009)。

[6]“Detecting sp² phase on diamond surfaces by atomic force microscopy phase imaging and its effect on surface conductivity”,Kozak et al,Diamond&related materials,18,722-725,(2009)。

Claims

1. Method for machining diamond by means of a pulsed laser, according to which:

2. The method of claim 1, wherein the transparent liquid comprises a surfactant additive.

3. The process according to claim 2, wherein the surfactant additive is of the polyoxyethylene type.

4. The method according to claim 1, wherein the transparent liquid further comprises a mineral salt in an amount of at most equal to 10% by mass.

5. The method according to claim 4, wherein the mineral salt is sodium chloride.

6. The method according to claim 1, wherein the transparent liquid is moved by mechanical agitation.

7. The method according to claim 1, wherein the level of transparent liquid above the surface to be processed is at least equal to 100 microns and at most equal to 3 mm.

8. The method of claim 1, wherein the pulse duration is from 100 femtoseconds to 100 picoseconds for a repetition frequency of 100-.

9. The method as claimed in claim 1, wherein the laser beam has a wavelength of 500-530 nm.

10. The method of claim 1, wherein the scanning is controlled axially transverse to the laser beam and between two transverse scan cycles in steps up to equal 10 microns.

11. The method of claim 1, wherein the polarization of the laser light is circular.

12. The method of claim 1, wherein the processing is performed in one or more cycles to a depth of at least 10 microns.

13. The method according to any one of claims 1 to 12, wherein the focusing objective is an immersion objective directly immersed in the transparent liquid used.